Dynode for crossed field electron multiplier devices

ABSTRACT

The secondary electron emission surface portion of the dynode electrode of a crossed field electron multiplier is provided with a concave surface facing the secondary electron stream for focusing the secondary electron stream against unwanted transverse spreading thereof to enhance collection efficiency and in some cases bandwidth.

United stateS Patent [1 1 [111 3,757,157

Enck, Jr. et al. 1 Sept. 4, 1973 [54] DYNODE FOR CROSSED FIELD ELECTRON 3,388,282 6/1968 Hankin et al. 1. 313/103 X MULTIPLIER DEVICES 3,649,868 3/l972 Bensussan... 3l5/39 3,593,058 7/1971 Hogg 315/39 [75] Inventors: Richar 8- Buck, J M n m 3,233,140 2/1966 Holshouser 313/103 X View; Wayne G. Abraham, Los 3,573,464 4/1971 Miya 313/103 X Altos Hills, both of Calif. [73] Assignee: Varian Associates, Palo Alto, Calif. Primary [hammer-Rudolph Rounec Assistant Exammer-Saxfield Chatmon, Jr. Flledi 1971 Attorney-Stanley Z. Cole, Robert K. Stoddard et al.

[21] Appl. No.: 202,541

[57] ABSTRACT 52 US. Cl 315/39, 313/103, 315/393 The secondary electron emission Surface Portion of the 51 1111. C1 1101 j 7/46, H01 j 19/80 dynode electrode of a crossed field electron multiplier [58] Field of Search 313/103, 104, 105; is Provided with a concave Surface facing the Secondary 315/39, 39.3 electron stream for focusing the secondary electron stream against unwanted transverse spreading thereof [56] Referen Cit d to enhance collection efficiency and in some cases UNITED STATES PATENTS bandw'dth- 3,431,420 3/ 1969 Fisher 314/393 X 4 Claims, 11 Drawing Figures sum 1M2 .l NANO SEC. 6

|.0 NANO SEC.

HP'I

N2 I844 r1 FIG.60

FIG.2

FIG.5

PAIENTEDSEP 4m:

mam

FIGS

DYNODE FOR CROSSED FIELD ELECTRON MULTIPLIER DEVICES GOVERNMENT CONTRACT The invention herein described was made in the course of or under a contract or subcontract with the department of defense.

DESCRIPTION OF THE PRIOR ART Heretofore, the secondary electron emissive surface portion of the dynode structure in a crossed field electron multiplier device has been flat. The result of a flat secondary emissive surface portion of the dynode is that the secondary electrons, as emitted from the dynode, tend to spread out transversely to the direction of the secondary electron stream. As a result, the collection efficiency is reduced in high frequency electron multipliers because the collector, to reduce collector capacitance for high frequency response, should be as small as possible. Thus when utilizing a small collector for high frequency response, spreading of the secondary electron stream reduces collection efficiency because a large portion of the secondary electron stream is not collected on the small collector plate.

Another disadvantage of spreading of the secondary electron stream is that, to the degree that divergent electrons are collected, such divergent electrons traverse a longer path length than the path length for the electrons at the center of the electron stream. As a consequence, the difference in traversed electron path length to the collector produces a spreading of the transit times for the secondary electrons, such that the rise time for a pulse being multiplied is increased, thereby decreasing the ability of the electron multiplier to discriminate between pulses of short duration.

Generally speaking, in the present state of the art, the rise time of a static multiple dynode crossed field electron multiplier is approximately 1 nanosecond. This translates to the frequency domain as a high frequency use cutoff of approximately l GHz. A similar problem is encountered in a dynamic crossed field multiplier wherein the dynode is a continuous secondary electron emitter surface over which a radio frequency electric field is produced at right angles to a static magnetic field for causing certain of the electrons in phase with the applied electric field to traverse cycloidal multiple hop trajectories down the length of the multiplier. In such a multiplier, transverse spreading of the secondary electron stream reduces the collection efficiency to on the order of percent. Such a multiplier is disclosed in an article titled, Subnanosecond Gating Properties of the Dynamic Cross-Field Photomultiplier, appearing in the Proceedings of the IEEE, Vol. 58, No. 10, October, 1970, pgs. 1487-1490.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved dynode structure for crossed field electron multiplier devices.

In one feature of the present invention, the secondary electron emissive surface of a dynode electrode in a crossed field electron multiplier device is formed with a concave surface facing the electron stream. The concave surface has an axis of revolution parallel to the direction of the secondary electron stream for focusing the secondary electron stream against unwanted transverse spreading thereof, whereby the collection efficiency of the electron multiplier is enhanced.

In another feature of the present invention, the dynode electrode includes a plurality of insulated dynode segments successively arranged along the path of the multiplied secondary electron stream, each of the dynode segments having a concave secondary electron surface facing the electron stream, with an axis of revolution parallel to the direction of the secondary electron stream.

In another feature of the present invention, the electron multiplier is of the dynamic type and includes means for applying radio frequency electromagnetic wave energy to the region over the secondary electron emissive surface of the dynode.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a static crossed field electron multiplier incorporating features of the present invention,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 22 in the direction of the arrows,

FIG. 3 is an enlarged perspective view of a dynode portion of the structure of FIG. 1 delineated by line 3-3,

FIG. 4 is an enlarged sectional view of a portion of the structure of FIG. 1 delineated by line 4-4,

FIG. 5 is an enlarged sectional view of a portion of the structure of FIG. 4 taken along line 5-5 in the direction of the arrows,

FIGS. 6(a) and 6(b) are waveforms of input and output pulses for the prior art electron multiplier,

FIG. 7 is a longitudinal sectional view of a dynamic crossed field electron multiplier incorporating features of the present invention,

FIG. 8 is a sectional view of the structure of FIG. 7 taken along line 8-8 in the direction of the arrows,

FIG. 9 is a transverse sectional view of the structure of FIG. 7 taken along line 9-9 in the direction of the arrows, and

FIG. 10 is an enlarged detailed view of a portion of the structure of FIG. 7 delineated by line l0-l0.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is'shown a static crossed field electron multiplier l (photomultiplier) incorporating features of the present invention.

The electron multiplier 1 includes an evacuated metallic envelope 2, as of the Ni-Cu-Mn-Fe alloy sold under the trademark Monel, having a length l, as of 4.5 inches, a height h, as of 2.0 inches, and a depth d, as of 1.0 inch. The envelope is generally of rectangular shape, having a pair of broad parallel front and back sidewalls 3 (FIG. 2), 4 and a pair of narrow rectangular top and bottom walls 5 and 6 and end walls sealed to the front and back sidewalls 3 and 4 via mating flange portions 7.

A magnet, only partially shown in FIG. 2, is disposed with its north and south poles adjacent the front and back sidewalls 3 and 4 of the enclosure 2 for producing a uniform static magnetic field B, as of 200 gauss, within the envelope 1, such field being perpendicular to and passing through the front and back sidewalls 3 and 4, respectively.

A segmented dynode electrode structure 8 is disposed within the envelope 2. The segmented dynode electrode structure 8 comprises a series of dynode electrodes 9 supported within the envelope 2 by feedthrough insulator assemblies 11 passing through the bottom wall 6 of the envelope 2. A substantially elongated planar electrode 12 (rail) is supported from the top wall of the envelope 2 by a plurality of feedthrough insulators 113.

The upper rail electrode 12 serves to establish a unipotential surface for cooperation with the potential established on the individual dynode electrodes 9 to establish a uniform electric field in the space between the dynodes 9 and and the upper electrode 12. In a typical example, the upper electrode 12 or rail is operated at a relatively high positive potential, as of +l,000 volts, relative to ground, whereas the individual dynode electrodes 9 are operated at successively lower potentials, as of 400, 800, l,200 and l,600 volts, respectively moving from right to left, as indicated on the drawing. The spacing between the successive electrodes 9 and the rail electrode 12 is selected such that the plane of the upper surface of each of the respective dynodes lies on an equipotential line of the electric field between the rail electrode and the dynode structure corresponding to its potential.

A photocathode electrode or input electrode 15 is disposed at the input or left end of the array of dynodes and is supported from the bottom wall 6 of the envelope 2 by a feedthrough insulator 111. The photocathode is disposed such that its upper photocathode surface is on an equipotential line corresponding to the operating potential of the photocathode surface 15, as of 2,000 volts.

An optically transparent window 16 is disposed over an aperture 17 in the top wall 5 of the envelope 2 directly over the photocathode 15. The rail electrode 12 has a gridded aperture 118 in registration with the window 16 to provide an optically transparent path through the window and onto the photocathode 15 for the passage of input light to be detected.

A collector electrode assembly 21 is mounted from the right hand end wall 22 of the envelope 2, such collector electrode structure extending axially of the envelope 2 with an upper surface 23 thereof (H6. 4) being disposed at an equipotential of the electric field corresponding to the operating potential of the collector electrode 21, which is ground potential.

A disc-shaped collector plate 24, as shown in FIG. 5, is disposed within a generally rectangular cavity 29 in collector electrode 21, slightly below the upper surface 23 thereof. A gridded aperture 25 is disposed over the collector plate 24, such gridded aperture 25 serving to maintain the electrostatic equipotential line of the upper surface 23 of the collector electrode assembly 21. The collector plate 26 is supported upon the end of a center conductor 26 of a coaxial line 27 and the outer end of the coaxial line is sealed in a gas-tight manner by a radio frequency window 28 terminating in a conventional coaxial connector, not shown.

The dynode electrodes 9 each comprise a metallic channel support or hat member 31, as of Monel, having a length, as of 0.550 inch, and a width, as of 0.75 inch. The channel member includes a lip or longitudinally directed flange portion 32 (See FIG. 3) which is fixedly secured to a similarly flanged concave secondary emitter member 33 as of beryllium copper or gallium phosphide. In a typical example, the secondary emitter 33 comprises a thin cylindrical section of beryllium copper, as of 0.020 inch thick and having a length of 0.550 inch with outer flange portions 34 joined to the support channel 31 via spotwelds along the mating flange portions 34 and 32. In a typical example, the concave upper surface of each of the dynode electrodes has a radius of curvature as of 0.5 to 1.0 inch with the axis of revolution of the secondary emissive surface being parallel to the longitudinal axis of the envelope 2 and to the secondary electron stream, as described below. The envelope and all of the electrodes and metallic members within the envelope are made of a nonmagnetic material, as of Monel, such as not to disturb the uniformity of the magnetic field B within the envelope 2.

The photocathode 15 includes a channel shaped support or hat 31, of the same configuration as those of the dynodes 9, which in-turn supports a concave photocathode 36, such as a JETEC (Joint Electron Tube Engineering Counsel) type S-2O photocathode. Photocathode 36 is concave with the same radius of curvature as the dynodes 9, and with the axis of revolution for the concave cylindrical photocathode surface being parallel to the longitudinal axis of the dynode array and being on the midplane of the dynode array, as are all the remaining axes of revolution for the successive cylindrical secondary emission surface portions of the successive dynode segments 9.

In operation, photons of light within the wavelength range to which the photocathode 36 is sensitive are shone through the window l6 and gridded aperture 18 onto the photocathode 36. Primary electrons emitted from photocathode 36, under the influence of the crossed electric and magnetic fields, are caused to execute a cycloidal trajectory or hop from the photocathode 36 to the first dynode 9 and thence via successive hops over the dynodes from one dynode to the next and thence through the grid 25 of the collector 21 onto the collector plate 24 at ground potential. Because each of the successive dynodes 9 is at a more positive potential, the secondary electron stream bombards each successive dynode with substantially increased energy to produce copious secondary emission on each hop. Thus, a bundle of primary electrons released from the photocathode 36, when incident upon the first dynode, produces a corresponding bundle of secondary electrons which hops to the next successive dynode to produce even more electrons. In this manner, the bundle of electrons, in traversing the dynodes 9, increases in secondary electron current. The final bundle is collected on the collector plate 24 to produce output current. In a typical example of a photomultiplier as shown in FIG. ll, the gain K of the electron multiplier is between 10 and 10.

Due to the concave curvature of the individual secondary electron emissive surfaces 33 of the successive dynodes 9, the secondary electrons are focused toward the longitudinal midplane of the dynode array such that undesired beam spreading is avoided. In this manner, the entire secondary stream is focused to a narrow ribbon to be entirely collected on the collector plate 24, except for that portion of the secondary electron stream which is intercepted by the collector grid 25. The collector grid 25 typically has an electron transparency on the order of 80 percent or more such that the collector efficiency is 80 percent or more in use.

Another advantage do the concave curvature of the secondary emissive surfaces 33 of the dynodes 9 is that, since the electrons are focused into a midplane ribbon, they experience substantially the same path length along the array of dynodes to the collector 24 such that the transit time for the electrons within a given bundle is substantially the same as compared to an electron stream which is allowed to spread. In a spreading secondary electron stream, outer side edges of the stream traverse longer path lengths to the collector plate 24 than the multiplier those electrons lying in the midplane of the secondary electron stream. As a consequence, the rise time for a spreading beam electron multiplier is substantially increased as compared to that of a narrow beam tube. More particularly, in the prior art, an input pulse of electrons having an amplitude or pulse shape as shown in FIG. 6(a) with a pulse width of approximately 0.1 nanosecond produced an output response or output pulse having a waveform shape as shown in FIG. 6(b). In FIG. 6(b) it is seen that the original pulse width of 0.1 nanosecond resulted in an output pulse having a width of approximately 1 nanosecond between percent points. This output pulse was also characterized by a rise time of approximately 1.0 nanosecond. The bandwidth of the multiplier is approximately equal to one over the minimal rise time which for the prior art is approximately 1 GHz. However, by utilizing the concave secondary emitting surface portions 33 of the dynodes 9, the minimum rise time has been reduced by a factor of 5, thereby resulting in a five times increase in the effective bandwidth of the multiplier to produce a bandwidth of approximately 5 GI-lz.

Referring now to FIGS. 7-10, there is shown a dynamic crossed field electron multiplier 41 (photomultiplier) employing alternative features of the present invention.

The dynamic crossed field photomultiplier 41 of FIGS. 7-10 is similar to that previously described with regard to FIGS. 1 and 2 with the exception that the static electric field is augmented by a radio frequency electric field and the dynode electrode comprises a single d.c. potential electrode structure extending longitudinally of the envelope; More particularly, the photomultiplier 41 comprises an elongated cup-shaped envelope 42 as of Monel having a top cover 43 sealed to the cup 42 about the flanged lip thereof at 44 to form an evacuated envelope structure. The envelope 42 is joined to a base plate 45 and an elongated rectangular dynode 46 is disposed within the envelope 42 along the bottom wall thereof and spaced therefrom by means of a sheet of insulative material as of alumina 0.010 inch thick. A plurality of metallic tabs 47 hold the secondary emissive dynode 46 to the bottom wall of the envelope 42. Additional sheet-shaped segments of insulative material 40 are provided between the dynode 46 and the tab 47 to allow independent electrical potentials to be applied therebetween, as of --300 v, to the dynode 46 relative to the grounded envelope 42.

A metallic top rail plate electrode 48 is supported over the dynode 46 in parallel relation thereto via the intermediary of a pair of end supports 49 serving to support the rail electrode -48 from the top cover 43 of the envelope and forming tumed-up end portions thereof shorted at the end wall 43. In a typical example,

the top rail 48 is spaced by 0.125 inch from the dynode 46. The dynode 46 and rail 48 have a length as of 3 inches and a width as of 0.5 inch. The rail 48, as supported within the enclosure 42, forms a section of coaxial line shorted at its ends to form a resonator and excited with electromagnetic energy via an input RF connector assembly 51 coupled to the coaxial line via input loop portion 52.

The resonant section of coaxial line is tuned for a resonant frequency near the frequency of excitation. In a typical example, the length of coaxial line is dimensioned for resonance at approximately 1 GHz and RF energy at 1 GHz is coupled to the cavity via input connector 51 for exciting resonance at 1 GHz. A tuner assembly 53 is provided intermediate the length of coaxial cavity with a transversely translatable metallic tuning block 54 protruding into the resonator for tuning thereof. The block 54 is supported from the top wall 43 via a deformable diaphragm 55 and is translated by means of a jack screw 56 carried from the cover plate 43 by a support arm 57.

A photocathode 58, such as an S-20 photocathode, is disposed at the upstream end of the dynode 46 as a portion thereof and an electron collector plate 59 is disposed at the downstream end of the cavity in the bottom wall thereof. More particularly, the dynode 46 includes a centrally disposed circular aperture 50, as of 0.25 inch in diameter, in the bottom wall thereof. The aperture is closed off via a double screen mesh 62 and the collector disc 59 is supported upon the inner end of the center conductor of a coaxial line 63 forming the output RF coaxial line connection.

The double screen mesh 62 is brazed across the opposite sides of a washer 64 to prevent leakage of radio frequency energy from the cavity to the coaxial line 63 (See FIG. 10). Collector disc 59 is operated at ground potential, whereas the insulated dynode 46 may be operated at a potential different than ground to provide a uniform DC electric field between the rail 48 and the dynode 46. A magnet having pole faces 65 and 66 is disposed on opposite sides of the envelope 42 for applying a uniform magnetic field B through the region of space between the rail 48 and dynode 46, such magnetic field extending parallel to the lower surface of the rail 48 and being uniform throughout the region of the coaxial line between the rail 48 and dynode 46.

An optically transparent window 68 is sealed over an aperture inthe top wall 43 of the envelope and a gridded aperture 69 is provided in the rail 48 above the photocathode portion 58 of the photomultiplier 41.

The dynode 46 is provided with a concave secondary emitter surface 71 facing the rail48. The radius of curvature of the concave secondary emitting surface 71 is such as to focus the secondary electron stream into a ribbon extending along the longitudinal midplane of the region of space between the rail 48 and dynode 46. In a typical example, the concave secondary emitting surface 71 has a radius of curvature as of 0.5 to 0.75 inch and the axis of revolution of the concave surface is parallel to the longitudinal axis of the secondary electron stream and substantially in the midplane of the dynode electrode structure. The photocathode 58 is curved in the same manner as the dynode 46 to provide focusing of the primary or photoelectrons emitted therefrom.

In operation, a photon of optical energy within the spectral range of the photocathode 58 passes through the input window 68, apertured rail 48, and onto the photocathode 58 for producing primary electron emission. The primary electron emission comes under the influence of crossed DC electric and DC magnetic fields plus a superimposed radio frequency electric field parallel to the DC electric field in the region between the rail 48 and the photocathode 58 and dynode 46. Under the influence of the crossed electric and magnetic fields, primary electrons, of the proper phase compared to the phase of the applied alternating electric field, will execute cycloidal hops, causing the primary electrons to back-bombard the dynode 46 to produce secondary electron current. More particularly, if the electric field is positive, the bundle of electrons will be accelerated from the photocathode toward the rail 48. At the same time, the magnetic field will be causing electrons to curve into a path returning the electrons to the dynode 46 with some translation along the longitudinal axis of the rail, that is from left to right. If the alternating electric field conditions are right, the electron bundle will return to the dynode 46 with enough energy to cause secondary emission with a secondary emission ratio 8 greater than one. The secondary electrons then repeat this process until after N hops the total translation is the length of the dynode 46 to the collector plate 59 and the electrons are collected. Each hop of the electrons increases the number of secondary electrons by the secondary emission ratio, giving a gain K of 8".

If the electric field has only a DC component, any energy gain moving away from the dynode 46 would be lost in returning to the dynode 46, thus the arrival energy would be just equal to the electrons initial energy.

For both secondary electrons and primary photoelec-' trons, the initial energy is too low to produce secondary emission ratios greater than one. For this reason, the electric field contains the RF component. The RF component allows the electric field to be greater while the electron is moving away from the dynode 46 than during its return. This allows for a sufiicient energy gain so that the electrons have enough impact energy upon returning to the dynode for good secondary emission. However, due to this RF field, the electric field an electron experiences during a multiplication step will demore, allowing them to overtake the electrons which were produced earlier/The effect is to cause a bunching of the electrons in time (or phase). This packet of electrons is further bunched and multiplied on succeeding steps, finally producing an output which instead of being continuous is a series of pulses, each one hopefully corresponding to the amount of light which fell during the previous RF cycle. The sampling frequency is thus equal to the RF drive frequency and the sampling time is some portion of the RF period. The sampling is not uniform over each RF period but is weighted by the sampling function, which in turn depends upon the field strengths.

Some small component of the RF field will leak through to the output coupling line 63 and thus synchronous detection is preferably employed with a small buckout to cancel the undesired coupling of RF energy through the output screen 62 to the output coaxial line 63. In a typical example, the electrons experience between 11 and 14 hops in traversing the length of the dynode 46 from the photocathode 58 to the collector 59.

As in the previous example, the concave cylindrical curvature to the secondary emitting surface of the dynode 46 causes the secondary electron stream to be focused into a ribbon rather than spreading, whereby the collector efliciency is substantially increased compared to a prior art electron multiplier utilizing a planar dynode 46. In a typical example, the dynamic electron multiplier 41 has a gain K between 10 and 10 and provides a collector efficiency equal to or greater than percent, such collector efficiency being limited only by the transparency of the collector screen 62.

Although the electron multipliers of the present invention have been explained utilizing a photocathode as the source of primary electrons, this is not a requirement. As an alternative, the electron multiplier portion of the photomultipliers may be utilized for multiplying the electron current of an input beam of electrons or ions directed onto the first dynode or upstream end of the dynode.

What is claimed is:

1. In a crossed field electron multiplier, evacuated envelope means, dynode electrode means within said envelope, said dynode electrode means having a secondary electron emissive surface portion for bombardment by electrons to produce copious secondary electron emissions therefrom, means for establishing crossed electric and magnetic fields within said envelope over said secondary electron emissive surface of said dynode electrode means for causing the secondary electron emission to back bombard said secondary emissive surface portion of said dynode electrode means, means for producing an input stream of primary electrons to be multiplied and for directing the primary electron stream against said secondary electron emissive surface of said dynode electrode means to produce a stream of secondary electrons of multiplied electron current, collector means for collecting the multiplied stream of secondary electron current for deriving an output, THE IMPROVEMENT WHEREIN, said dynode electrode means has a secondary electron emissive surface for bombardment by electrons to be multiplied, said surface having a concave curved portion facing said electron stream, said curved portion having but a single axis of rotation generally oriented along the length of said electron stream such that the outer edge portions of said dynode electrode means are higher than any intermediate portion thereof, said curved portion extending across at least most of the width of said dynode electrode means, such that said dynode electrode means will focus said stream into a single ribbon, thereby to obviate unwanted transverse spreading thereof.

2. The apparatus 0F claim 1 wherein said dynode electrode means includes'afplurality of dynode segments successively arranged along the path of the multip lied secondary electron stream, and means for electrically insulating said successive dynode segments from each other to allow different electrical potentials to be applied thereto in use.

3. The apparatus of claim I wherein said means for applying the crossed electric andmagnetic fields over said dynode electrode includes means for applying electromagnetic wave energy to the region over said secondary electron emissive surface portion of said dynode electrode means, said electromagnetic wave being applied with the electric field vector thereof being generally normal to the secondary electron emissive surface portion of said dynode electrode means.

4. The apparatus of claim 1 wherein said concave secondary electron emissive portion of said dynode electrode is generally a cylindrical section. 

1. In a crossed field electron multiplier, evacuated envelope means, dynode electrode means within said envelope, said dynode electrode means having a secondary electron emissive surface portion for bombardment by electrons to produce copious secondary electron emissions therefrom, means for establishing crossed electric and magnetic fields within said envelope over said secondary electron emissive surface of said dynode electrode means for causing the secondary electron emission to back bombard said secondary emissive surface portion of said dynode electrode means, means for producing an input stream of primary electrons to be multiplied and for directing the primary electron stream against said secondary electron emissive surface of said dynode electrode means to produce a stream of secondary electrons of multiplied electron current, collector means for collecting the multiplied stream of secondary electron current for deriving an output, THE IMPROVEMENT WHEREIN, said dynode electrode means has a secondary electron emissive surface for bombardment by electrons to be multiplied, said surface having a concave curved portion facing said electron stream, said curved portion having but a single axis of rotation generally oriented along the length of said electron stream such that the outer edge portions of said dynode electrode means are higher than any intermediate portion thereof, said curved portion extending across at least most of the width of said dynode electrode means, such that said dynode electrode means will focus said stream into a single ribbon, thereby to obviate unwantEd transverse spreading thereof.
 2. The apparatus oF claim 1 wherein said dynode electrode means includes a plurality of dynode segments successively arranged along the path of the multiplied secondary electron stream, and means for electrically insulating said successive dynode segments from each other to allow different electrical potentials to be applied thereto in use.
 3. The apparatus of claim 1 wherein said means for applying the crossed electric and magnetic fields over said dynode electrode includes means for applying electromagnetic wave energy to the region over said secondary electron emissive surface portion of said dynode electrode means, said electromagnetic wave being applied with the electric field vector thereof being generally normal to the secondary electron emissive surface portion of said dynode electrode means.
 4. The apparatus of claim 1 wherein said concave secondary electron emissive portion of said dynode electrode is generally a cylindrical section. 